It will travel 21 miles upstream in three hours going down stream we can show 39 miles in the same amount of time find the speed of the current in the speed of the boat in still water

The speed of the current is 3 miles per hour.

The speed of the boat in still water is 10 miles per hour

Step-by-step explanation:

Let the speed of the boat be x miles per hour

Let the speed of the current be y miles per hour

Speed = distance / time

If the boat moves 39 miles downstream in 3 hours, then, the speed is

39/3= 13 miles per hour

Let us assume that it moved it moved in the same direction with the current hence, it moved in still water

The total speed would be x + y. Therefore

x + y = 13 - - - - - - -1

If the boat moves 21 miles upstream in 3 hours, the the speed is

21/ 3 = 7 miles per hour

It is assumed that it moved in the opposite direction to the current.

The total speed would be x - y. Therefore

x - y = 7 - - - - - - -2

Adding equation 1 and equation 2, it becomes

2x = 20

x = 20/2 = 10 miles per hour

y = 13 - x

y = 13 - 10 = 3 miles per hour